1. Field of the Invention
The invention relates to an analog-to-digital signal converter comprising two or more one-bit sigma-delta modulators, these modulators comprising each a closed loop arrangement of at least an adder, a low-pass filter and a pulse shaping circuit driven with a specific sampling frequency.
2. Description of the Related Art
A signal converter of this type is known from the article by Y. Matsuya et al. entitled: "A 16-bit Oversampling A-to-D Conversion Technology Using Triple-Integration Noise Shaping" published in IEEE Journal of the Solid-State Circuits, Vol. Sc-22, No. 6, Dec. 1987, pp. 921-929.
In the prior-art signal converter the noise shaping is realized by means of three sigma-delta modulators for working with a relatively low sampling frequency. In each of the sigma-delta modulators the selectiveness of the low-pass filter is determined by an integrator arranged as a first-order switched capacitor integrator. With the applied oversampling factor (64.times.) a signal-to-noise ratio of about 50 dB (8 bits) is attained in the first sigma-delta modulator. The remaining noise is largely compensated for with the aid of the next sigma-delta modulators so as to attain, after addition in the decimators, a signal-to-noise ratio in excess of 90 dB. Therefore, the matching of the various branches is to be better than 1%.
The prior-art analog-to-digital converter is unsuitable for video applications which require signals having a large to a very large number of bits.